Fix missing return checks in v3_cpols.c

Fixed assorted missing return value checks in v3_cpols.c

(Imported from upstream's c5f2b5336ab72e40ab91e2ca85639f51fa3178c6.)

Change-Id: I95e4157fc689201ca32e1c8e51d04801ba478685
Reviewed-on: https://boringssl-review.googlesource.com/4008
Reviewed-by: Adam Langley <agl@google.com>